Inverters typically develop an ac output voltage from a dc source, such as a battery stack, by driving a low-pass filter with a PWM (pulse width modulator) operating at a switching frequency that is much higher than the frequency of the required ac output voltage. The ac output voltage typically has a fixed frequency and therefore a fixed period. An ac inverter with a 60 Hz output frequency will typically have a PWM driving the low pass filter at a frequency of 5 KHz or above. A transformer is conventionally used to achieve isolation between the dc source and the load driven by the ac output voltage.
The invention crest factor correction circuit is used with an ac inverter circuit to vary the duty ratio of the PWM a fixed number of times during each ac output voltage period. The control voltage for controlling the duty ratio of the pulse width modulator in a control system of this type is typically a stepwise or staircase approximation of a sinusoid referred to as a composite control signal. Typical feedback control is achieved by controlling the average or RMS signal of the ac inverter output, or a rectified version of the inverter output.
The amplitude of the composite control signal typically remains constant during each phase interval of each ac output voltage cycle. The constant amplitude of the composite control signal during each phase interval fixes the duty ratio of the PWM during that phase interval. The ac output voltage of the ac inverter is controlled by varying the overall amplitude of the staircase envelope either by average or RMS control.
The loads driven by inverters using the above processes sometimes demand a current only during a short phase interval at the crest interval or peak phase interval of the ac output voltage. Equipment having capacitor rectifier power input stages, such as personal computers, provide a load of this character. Ac inverters using the above control process are capable of adjusting the duty ratio of the PWM only as an average or RMS control, but such adjustments, while perhaps increasing the average or RMS value, may actually flatten the crest value. For high impedance output inverters, a subsequent increase in load will further reduce the peak inverter output.
Commercial electronic equipment operating on the ac output voltage of an ac inverter typically monitor the amplitude of the peak swing or crest of the ac output voltage for the purpose of determining if the power is suitable for continued operation. In the average or RMS feedback control as described above, the probability of inadvertent equipment shut down is increased as workstations, including added rectifier capacitive loads, are placed on the line and as crest voltage clipping increases.